1. Field of the Invention
The invention relates generally to the field of imaging rock formations. More specifically, the invention relates to systems and methods for obtaining neutron images of a rock formation with a tool that need not rotate while drilling and/or measuring.
2. Background Art
Neutron tools have been used for several decades to measure the neutron porosity and hydrogen index of earth formations. Modern tools use pulsed neutron sources and thermal and/or epithermal neutron detectors for the measurement of the neutron flux of the neutrons at several distances from the neutron source. Additionally, the neutron slowing down time measured by one or more of the detectors is a shallow measurement of hydrogen index and very sensitive to standoff. The traditional porosity measurement relies on deriving liquid filled porosity from the ratio of the neutron fluxes from at least two different distances from the source.
These neutron tools are used widely in the petrochemical industry, particularly during the so-called LWD (Logging While Drilling) or MWD (Measurement While Drilling) stage, but also at other stages such Wireline. LWD/MWD is logging during the initial stage of drilling a hole down into the earth's crust typically towards an identified hydrocarbon reservoir, which should eventually form a producing oil or gas well for fulfilling energy needs. FIG. 1 is an illustration of an exemplary wellsite system, according to an exemplary embodiment. The wellsite can be onshore or offshore. In this exemplary system, a borehole 11 is formed in subsurface formations by rotary drilling in a manner that is well known.
A drill string 12 is suspended within the borehole 11 and has a bottom hole assembly 100 which includes a drill bit 105 at its lower end. The surface system includes platform and derrick assembly 10 positioned over the borehole 11, the assembly 10 including a rotary table 16, kelly 17, hook 18 and rotary swivel 19. The drill string 12 is rotated by the rotary table 16, energized by means not shown, which engages the kelly 17 at the upper end of the drill string. The drill string 12 is suspended from a hook 18, attached to a traveling block (also not shown), through the kelly 17 and a rotary swivel 19 which permits rotation of the drill string relative to the hook. As is well known, a top drive system could alternatively be used.
In the example of this embodiment, the surface system further includes drilling fluid or mud 26 stored in a pit 27 formed at the well site. A pump 29 delivers the drilling fluid 26 to the interior of the drill string 12 via a port in the swivel 19, causing the drilling fluid to flow downwardly through the drill string 12 as indicated by the directional arrow 8. The drilling fluid exits the drill string 12 via ports in the drill bit 105, and then circulates upwardly through the annulus region between the outside of the drill string and the wall of the borehole, as indicated by the directional arrows 9. In this well known manner, the drilling fluid lubricates the drill bit 105 and carries formation cuttings up to the surface as it is returned to the pit 27 for recirculation.
The bottom hole assembly (BHA) 100 of the illustrated embodiment comprises a logging-while-drilling (LWD) module 120, a measuring-while-drilling (MWD) module 130, a rotary-steerable system and motor, and drill bit 105. The LWD module 120 is housed in a special type of drill collar, as is known in the art, and can contain one or a plurality of known types of logging tools. It will also be understood that more than one LWD and/or MWD module can be employed, e.g. as represented at 120A. (References, throughout, to a module at the position of 120 can alternatively mean a module at the position of 120A as well.) The LWD module includes capabilities for measuring, processing, and storing information, as well as for communicating with the surface equipment. As an example that relates to the present disclosure, the LWD module can include a nuclear measuring device or neutron tool to measure, for example, the porosity of the surrounding formation.
The MWD module 130 is also housed in a special type of drill collar, as is known in the art, and can contain one or more devices for measuring characteristics of the drill string and drill bit. The MWD tool further may include an apparatus (not shown) for generating electrical power to the downhole system. This may typically include a mud turbine generator powered by the flow of the drilling fluid, it being understood that other power and/or battery systems may be employed. In the present embodiment, the MWD module includes one or more of the following types of measuring devices: a weight-on-bit measuring device, a torque measuring device, a vibration measuring device, a shock measuring device, a stick slip measuring device, a direction measuring device, and an inclination measuring device.
FIG. 2 shows a logging-while-drilling nuclear device as disclosed in U.S. Pat. Re. 36,012, incorporated herein by reference, which utilizes an accelerator-based source, it being understood that other types of nuclear LWD tools can also be utilized as the LWD tool 120 or part of an LWD tool suite 120A. In FIG. 2, a drill collar section 1040 is shown as surrounding a stainless steel tool chassis 1054. Formed in the chassis 1054 to one side of the longitudinal axis thereof (not visible in this view) is a longitudinally extending mud channel for conveying the drilling fluid downward through the drill string. Eccentered to the other side of the chassis 1054 are a neutron accelerator 1058, its associated control and high voltage electronics package 1060 and a coaxially aligned near-spaced detector 1062. The near-spaced detector 1062 is primarily responsive to accelerator output with minimum formation influence. The detector 1062 is surrounded, preferably on all surfaces except that adjacent to the accelerator 1058, by a shield 1064 of combined neutron moderating-neutron absorbing material. The output of the near detector 1062 is used to normalize other detector outputs for source strength fluctuation. Located longitudinally adjacent to the near-spaced detector 1062 is a plurality or array of detectors, of which 1066a and 1066d are shown in this view. The detector 1066a is back-shielded, as shown at 1068a. The array includes at least one, and preferably more than one, epithermal neutron detector and at least one gamma ray detector, represented in this example at 1084, with shield 1086. One or more thermal neutron detectors can also be included. The above-referenced U.S. Pat. Re. 36,012 can be referred to for further details. The detector signals can be utilized to determine, inter alia, formation density, porosity, and lithology.
As may be understood to those of ordinary skill in the art having benefit of the present disclosure, in conventional LWD wellsite systems such as those described above with reference to FIGS. 1 and 2, the entire drill string 12 often rotates while the drilling operation is performed, whether by a kelly 17 system or a top drive system. Moreover, in conventional LWD wellsite systems that include a nuclear tool, such as the LWD tool 120 described above, the tool 120 being part of the BHA 100 also rotates during the drilling operations. Accordingly, in LWD nuclear, conventional imaging measurements are made possible due to the fact that the tool rotates in the borehole and an azimuthally focused measurement will therefore make an azimuthal scan of the surrounding formation as the tool rotates. In the absence of tool rotation, i.e. when the tool is sliding, no image can be acquired for these tools. Moreover, additional LWD systems have been developed in recent years in which the entire drill string may not rotate or rotate only very slowly, such as where there is a downhole motor and/or where coiled tubing drilling is used, signifying yet another deficiency with rotation-based neutron imaging in LWD applications.
Accordingly, there is a need in the art for methods and systems for neutron imaging that overcome one or more of the deficiencies that exist with conventional methods.